need help dealership cant answer

i have a 1999 chevy express 2500 5.7 it looses power after driving for about 5mins. it starts to cut out backfires and sometimes stalls out and is difficult to restart. it was at the chevy dealership for 3 days. the catalytic converters are new so is the fuel filter vaccum lines o2 sensors mass air flow sensor plugs and wires and fuel pump was replaced 2 years ago and we did check to make sure there was good gas flow. i am at a loss as to where to go next this has already cost over $1200 can you help me please:banghead:

Welcome to AF, realistically we are at a desk here and of course cannot get hands on look/see. If the dlrship had it for 3 days what was the problem, the condition didn't occur? Where was the 1200 spent, if at a shop, get your money back or have them
fix it, if they cannot your are still entitled to some consideration if all these parts were replaced to fix this issue.

Tip on going to the dealer
Ask that only a factory certified gasoline engine performance specialist work on your car.
Not all dealerships are concerned with how repairs are dispatched, and not all dealership techs bother to take advantage of the training offered by factory (the vast majority of which is paid training). If the dealer is uncooperative, ask for your money back and call around for a dealer that will accommodate you.

Have a shop do a leakdown test on the 5.7 to see that maynot be the problem or starting the whole ball of problems.